I am almost certain that TL must in fact read TI, and that this stands for Tarjeta de identidad.

In Colombia the standard national identity card is called the Cédula de Ciudadanía, abbreviated to C.C. For minors (under 18) there is a different document called the Tarjeta de Identidad (T.I.):

"El documento de identidad es llamado Cédula de Ciudadanía o C.C., para el caso de los ciudadanos colombianos mayores de edad. [...]
Existe una cédula de identidad para los menores de 18 años, llamada Tarjeta de Identidad (que carece de validez en los actos para los que sirve la cédula de ciudadanía) usada para fines de identificación, la cual puede ser tramitada a partir de los 7 años de edad."
http://es.wikipedia.org/wiki/Documento_de_identidad#Colombia

The actual number you have quoted in the discussion area, 871207-51911, is precisely the format of a TI number. See the numbers given in the following document; all the TI numbers begin with six digits (of which the first is always 8), then a hyphen, then a further five digits. Sometimes the group of five digits has a thousands separator after the first two; sometimes not. One of the TI numbers in this document actually begins 871207, like yours:
http://bolivar.sena.edu.co/comercioyservicios/archivos/APREN...

I presume, therefore, that this is the Tarjeta de Identidad number of the person whose bachiller certificate this is, and who is presumably under 18. It is certainly quite plausible that this piece of information should appear. To me, at least, the idea that it is a telephone number is rather less plausible (apart from the fact that TL is not a standard abbreviation for teléfono; it's usually TF.