Check the URL. Does it contain special characters or encoding?

Special Characters produce special encoding when converted to a URL. For example, a website with a page "Rewards & Offers" may get the "&" and display it as "%26" in the URL. 

(i.e. www.website/rewards-%26-offers) 

The URL shortener is operating within the IIS frame and the presence of "%26" is being read as encoding, so it encodes it again when using the URL Shortener and adds more characters to the URL. 

(i.e. www.website/rewards-%2526-offers) 

This would break the redirecting of the URL.

Is there a way around this?

There is no way around this. For the URL shortener to redirect to the correct site, the site owner will need to remove the "%26" from the URL. 

(i.e. Change "Rewards & Offers" to "Rewards and Offers") 

Alternatively, if updating the site URL is nor viable, consider sending the text as an MMS message and include the complete URL instead of a shortened URL.