An efficient method and system for detecting frame slip in an inband signalling block in pulse code modulation. The effect of frame slip on the inband signalling block is that the bits following the frame slip are transferred from the signalling block into an adjacent block. The octet slip is detected by searching an error bit in a signalling block by comparing it to a sample block. If an error bit is found, an error count for the adjacent block starting from the error bit is calculated. If the error count is more than one, a second error bit of the signalling block is searched (26) and bits of the adjacent block after second error bit are verified (27). If bits of the adjacent block after the second error bit are not correct, the octet slip cannot be assumed (29). Otherwise the octet slip can be assumed by analyzing error count and error bits.