Mapping Help

Hi,

I have a mapping requirement as follows-

Need to delete the extention number from telephone number.

The telephone number is comming in the following formats- 414-234-1212 and 414234121 and after this some extention number is comming.Need to truncate it.we can't take length into consideration as we have two formats for telephone number whose lengths differ.Please suggest some UDF that will do this.Thanks in advance.