Anyone sending you a text message would dial as they would for any international number (starting with + and then the full international number).
As you have been given a full international number, you simply just need to add + in front of the My RoamPass number. The same format is used for calling and sending texts to the My RoamPass.
Example: To text the My RoamPass number 44 ### ##### they would send it to +44 ### #####
You can also send text messages to the My RoamPass for free online. Use My RoamPass's free texting service.