PPP modem differences

[colin.helliwell at ln-systems.com]

I know this might not be an issue with MM, but as there's also a lot of
modem-savvy people here.. :)

I've got the same system trying to make a ppp connection via MM. I'm using
the same SIM card, and running
  mmcli -m 0 --simple-connect="apn=mobile.o2.co.uk"
  pppd nodetach call provider

On one modem, it works fine. But on the other, it doesn't [though it has
done, with some permutation of versions, in the recent past....]
The MM logs seem sensible and equivalent, but there are strange differences
in the ppp logs:

sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x90170814> <pcomp> <accomp>]
rcvd [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x6b7e4a17> <pcomp> <accomp>]
sent [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x6b7e4a17> <pcomp> <accomp>]
rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x90170814> <pcomp> <accomp>]
kernel does not support PPP filtering
sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15> <bsd v1 15>]
sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns2
0.0.0.0>]
rcvd [LCP ProtRej id=0x2 80 fd 01 01 00 0f 1a 04 78 00 18 04 78 00 15]
Protocol-Reject for 'Compression Control Protocol' (0x80fd) received
sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns2
0.0.0.0>]
rcvd [IPCP ConfReq id=0x1]
sent [IPCP ConfNak id=0x1 <addr 0.0.0.0>]
rcvd [IPCP ConfNak id=0x1 <addr 10.232.128.88> <ms-dns1 10.203.128.1>
<ms-dns2 10.203.128.1>]
sent [IPCP ConfReq id=0x2 <addr 10.232.128.88> <ms-dns1 10.203.128.1>
<ms-dns2 10.203.128.1>]
rcvd [IPCP ConfReq id=0x2 <addr 10.232.128.88>]
sent [IPCP ConfAck id=0x2 <addr 10.232.128.88>]
rcvd [IPCP ConfAck id=0x2 <addr 10.232.128.88> <ms-dns1 10.203.128.1>
<ms-dns2 10.203.128.1>]
not replacing default route to eth0 [192.168.100.1]
local  IP address 10.232.128.88
remote IP address 10.232.128.88
primary   DNS address 10.203.128.1
secondary DNS address 10.203.128.1

sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0xf16ed179> <pcomp> <accomp>]
rcvd [LCP ConfNak id=0x1 <asyncmap 0xa0000>]
sent [LCP ConfReq id=0x2 <asyncmap 0xa0000> <magic 0xf16ed179> <pcomp>
<accomp>]
rcvd [LCP ConfAck id=0x2 <asyncmap 0xa0000> <magic 0xf16ed179> <pcomp>
<accomp>]
rcvd [LCP ConfReq id=0x3 <asyncmap 0xa0000> <pcomp> <accomp> <magic
0x549d043f> <auth chap MD5>]
No auth is possible
sent [LCP ConfRej id=0x3 <auth chap MD5>]
rcvd [LCP ConfReq id=0x5 <asyncmap 0xa0000> <pcomp> <accomp> <magic
0x549d043f> <auth pap>]
No auth is possible
sent [LCP ConfRej id=0x5 <auth pap>]
rcvd [LCP ConfReq id=0x7 <asyncmap 0xa0000> <pcomp> <accomp> <magic
0x549d043f>]
sent [LCP ConfAck id=0x7 <asyncmap 0xa0000> <pcomp> <accomp> <magic
0x549d043f>]
kernel does not support PPP filtering
sent [CCP ConfReq id=0x1 <deflate 15> <deflate(old#) 15> <bsd v1 15>]
sent [IPCP ConfReq id=0x1 <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns2
0.0.0.0>]
rcvd [LCP ProtRej id=0x8 80 fd 01 01 00 0f 1a 04]
Protocol-Reject for 'Compression Control Protocol' (0x80fd) received
rcvd [LCP TermReq id=0x9]
LCP terminated by peer
sent [LCP TermAck id=0x9]
rcvd [LCP TermReq id=0x9]
sent [LCP TermAck id=0x9]
Connection terminated.
Modem hangup

I'm puzzled why, with the same software and SIM/provider/APN, the
non-working modem is 
a) receiving a "LCP ConfNak id=0x1 <asyncmap 0xa0000>]",  when the working
one gets an Ack to the same Req
b) receiving a "LCP ConfReq id=0x3 <asyncmap 0xa0000> <pcomp> <accomp>
<magic 0x549d043f> <auth chap MD5>" - or indeed why it's *receiving* a
ConfReq at all.

Are there characteristics in the modem which could alter how MM behaves? (Or
what the local and remote ppp's are sending/negotiating)
Any expertise welcome!