VPN client connection problems with Virgin Mobile BroadBand2Go Ovation MC760

[bucovey]

I have the VM 500Mb service with Virgin USB Broadband2Go.
My problem is I cannot get PPTP VPN connection. Nevertheless, I have an AT&T connection and I can use my VPN client.
I have seen this kind of problem in other forums with Virgin Devices. It seems some kind of restrictions, access list, MTU, profiles.
I am sending the logs of my VPN server, and I have noticed that is the same problem (same logs), LCP connection problems, described by other cortumers.

2012:02:09-09:38:18 firewall pptpd[15508]: CTRL: Client xxxxxx control connection finished
2012:02:09-09:46:25 firewall pptpd[15823]: CTRL: Client xxxxx control connection started
2012:02:09-09:46:25 firewall pptpd[15823]: CTRL: Starting call (launching pppd, opening GRE)
2012:02:09-09:46:25 firewall pppd-pptp[15824]: Plugin aua.so loaded.
2012:02:09-09:46:25 firewall pppd-pptp[15824]: AUA plugin initialized.
2012:02:09-09:46:25 firewall pppd-pptp[15824]: pppd 2.4.5 started by (unknown), uid 0
2012:02:09-09:46:25 firewall pppd-pptp[15824]: Starting negotiation on /dev/ttyp0
2012:02:09-09:46:25 firewall pptpd[15823]: GRE: Bad checksum from pppd.
2012:02:09-09:46:55 firewall pppd-pptp[15824]: LCP: timeout sending Config-Requests
2012:02:09-09:46:55 firewall pppd-pptp[15824]: Connection terminated.
2012:02:09-09:46:55 firewall pppd-pptp[15824]: Modem hangup
2012:02:09-09:46:55 firewall pppd-pptp[15824]: Exit.
2012:02:09-09:46:55 firewall pptpd[15823]: GRE: read(fd=6,buffer=805a540,len=8196) from PTY failed: status = -1 error = Input/output error, usually caused by unexpected termination of pppd, check option syntax and pppd logs
2012:02:09-09:46:55 firewall pptpd[15823]: CTRL: PTY read or GRE write failed (pty,gre)=(6,7)
2012:02:09-09:46:55 firewall pptpd[15823]: CTRL: Reaping child PPP[15824]
2012:02:09-09:46:55 firewall pptpd[15823]: CTRL: Client xxxxxx control connection finished

[GTwannabe]

PPTP VPN doesn't work over VM's network (probably due to the proxy they route our data through)