Ive been using a service at
www.telcan.net (NOT .com!) for a few years to do this. I have an 800 number there, and forward my TMo line to it. I then set Telcan to forward to my prepaid SIM. The caller pays whatever they would to call my TMo number, I use TMo minutes and the actual call to wherever is billed at the Telcan rates which are often way cheaper than roaming. Other services are available. There have been a lot of threads on this by the way. Skype is an interesting option also. But as mentioned, you cannot do it without an intermediate provider. TMo took that away a few years ago.