Target Line Issue

Sounds like software corruption, but here it is. MICS 7.1 w/PRI. No problems for two weeks since PRI was turned up & numbers ported. On Friday, three DID's stopped working. If you call any of the three DID's, the phone with the DID's assigned (appear & ring), gets a half of ring (time & date stays on the screen) and the caller gets a fast busy. Went to the customer site and the only way to fix problem was to move the info out of the current target lines (157, 164 & 165), to different target lines. I first moved the target lines to a different phone, still get the fast busy. Next removed info out of target line, got out of programming, went back into programming and put the info back in the target line (problem still exists). So, I fixed the problem by moving to other target lines, but whats up with the original target lines. Thanks.