PPP hangs-up before completing T-Mobile connection

Question

I'm trying to dial out on a GSM modem to a T-Mobile APN (epc.tmobile.com). However, pppd hangs-up before obtaining an IP address. Can anyone tell me why I'm not getting a connection with an IP address? Note, the GSM modem remains attached when ppp hangs-up. I can restart the pppd call multiple times. The ppp session looks like this:

-bash-4.0# pppd call tmobileGsm 
Starting Sierra Wireless GSM connect script... 
Setting the abort string 
Initializing modem 
Setting APN 
Dialing... 
Serial connection established. 
using channel 41 
Using interface ppp0 
Connect: ppp0 <--> /dev/ttySierraAT 
sent [LCP ConfReq id=0x1 <asyncmap 0x0> <magic 0x31a77a57> <pcomp> <accomp>] 
rcvd [LCP ConfReq id=0x27 <asyncmap 0x0> <auth chap MD5> <magic 0x730ab6a3> <pcomp> <accomp>] 
sent [LCP ConfAck id=0x27 <asyncmap 0x0> <auth chap MD5> <magic 0x730ab6a3> <pcomp> <accomp>] 
rcvd [LCP ConfAck id=0x1 <asyncmap 0x0> <magic 0x31a77a57> <pcomp> <accomp>] 
rcvd [LCP DiscReq id=0x28 magic=0x730ab6a3] 
rcvd [CHAP Challenge id=0x1 <4e8ec9fb06249e6d223e4fa904817891>, name = "UMTS_CHAP_SRVR"] 
sent [CHAP Response id=0x1 <54a5a4872d90fa1b7e2c85b8ce9debc7>, name = "scu1"] 
rcvd [CHAP Success id=0x1 ""] 
CHAP authentication succeeded 
CHAP authentication succeeded 
sent [IPCP ConfReq id=0x1 <compress VJ 0f 01> <addr 0.0.0.0> <ms-dns1 0.0.0.0> <ms-dns3 0.0.0.0>] 
Modem hangup 
Connection terminated. 
-bash-4.0# 

Here is tmobileGsm:

-detach 
/dev/ttySierraAT 
460800 
debug 
defaultroute 
usepeerdns 
noccp 
crtscts 
lock
connect '/usr/sbin/chat -v -t6 -f /etc/ppp/peers/tmobileGsm.connect.chat' 

Here is the options file:

lock 
noipdefault 

Here is the chap-secrets file:

# client        server  secret                  IP addresses 
* * * 

Here is my dial script:

# Generic T-Mobile PPPD script 
SAY 'Starting Sierra Wireless GSM connect script...\n' 
SAY '\n' 
SAY 'Setting the abort string\n' 
SAY '\n' 
# Abort String ------------------------------ 
ABORT 'NO DIAL TONE' ABORT 'NO ANSWER' ABORT 'NO CARRIER' ABORT DELAYED 

SAY 'Initializing modem\n' 
# Modem Initialization 
'' AT 
OK ATZ 

SAY '\n' 
SAY     'Setting APN\n' 

# T-Mobile Internet plan (all new plans) 
OK     'AT+CGDCONT=1,"IP","epc.tmobile.com"' 

SAY '\n' 
SAY     'Dialing...\n' 
# Dial the ISP, this is the common Cingular dial string 
OK ATD*99# 
CONNECT '' 

Any help is very much appreciated. Jim